Oh yes, DC, its a well-known scam - maybe not as well-known as it ought to be.
There's no simple answer to the original question. Very rough answers could be
agricultural land - �3000 per acre
'amenity land' - paddock for horses, �10k per acre
land on which PP can be or has been granted - depends entirely on the sales price potential for the houses when built. Always remember, its not the price of land that drives up the price of houses, its the other way around. The ability of buyers to pay for a house drives up the sales price - that drives up the bid price of land.